Peter Panum reported on a measles epidemic in the Faroe Islands in 1846; he noted that those individuals who had measles during the last epidemic, 65 years earlier, were spared. This was the first observation in the modern era that immunologic memory is long-lived. In 1968, the current live-attenuated measles vaccine was licensed and 30 years later, no indigenous cases of measles were detected in the U.S. The success of this vaccine underscores the necessity for developing vaccines for other pathogens for which long-lived immunologic memory is not apparent (e.g. influenza, respiratory syncytial virus and HIV). Toward this end, the goal of this proposal is to establish an animal model of long-term immunologic memory to measles vaccine in the rhesus monkey. Measles virus is pathogenic in rhesus monkeys and the immune responses of infected monkeys are similar to those in humans. Vaccination of monkeys with live attenuated measles vaccine protects them from systemic infection and disease after challenge with pathogenic measles virus. There are two aims and hypotheses for this proposal. 1. Memory B and T cells circulate in the blood and through body tissues differently than do naive cells, and patterns of Iymphocyte circulation and retention in tissues are related to the initial route of immunization. 2. The levels of memory B and T cells that persist during long-term memory are determined by the amount of viral antigen produced during the primary immune response.